A CPU typically has one or more cache memories arranged between the data and instruction inputs of its execution unit on the one hand and the port for connection to main memory. The caches compensate for the difference in speed between the processing in the CPU and the fetching of data and instructions from main memory. The successful operation of the cache relies on the locality principle: program references to memory tend to be clustered in time and in logical space. Temporal clustering relates to the tendency to reference the same address more than once within a specific period of time. Spatial clustering relates to the tendency to fetch data or instructions from logically consecutive memory addresses. The data and instructions in the main memory are mapped into the cache in blocks of logically coherent addresses. Below, the term "information item" is used to refer to either data or an instruction within this context.
A cache read miss occurs when the CPU requests an information item that is not present in its cache. The cache has thereupon to retrieve the appropriate block from the main memory or the secondary cache and store it. During this cache refill, the execution unit is stalled. Various techniques are in use to minimize the number of clock cycles that the execution unit has to idle as a result of a cache refill.
For example, European patent application 0 543 487 A1 discusses the early-restart technique. As soon as the requested item arrives from main memory it is sent to the execution unit without waiting for completion of the writing of the entire block to the cache. A refinement of this early-restart is the out-of-order fetch. The out-of-order fetch lets the main memory skip all information items located at addresses logically preceding the requested item in the relevant block. The requested item is sent directly to the execution unit upon retrieval while the remainder of the block is being retrieved looping around to fetch the items previously skipped.
European patent application 0 543 487 A1 also discusses an alternative technique that involves the following steps. If the CPU fetches data during a data cache fill and the requested data being fetched is part of the memory block being currently filled, the data is retrieved and returned to the execution unit simultaneously with its writing into the cache, if the data has not been written into the cache. If the data has been written into the cache, the data is retrieved and returned to the execution unit at the next read cycle.
Also see, e.g., "MIPS RISC Architecture", Gerry Kane and Joe Heinrich, Prentice Hall, 1992, notably Chapter 5, page 5-5. In the implementations of MIPS processor architectures, e.g., the R2000 and R3000, a typical sequence of events occurring after a cache miss are the following. On a cache miss, the processor reads one word from memory and stalls while the designated blocks in the cache are refilled. After the refill has been completed, missed information items are retrieved from the cache and are supplied to the processor's execution unit to resume processing. For general background information on the MIPS architecture, also see, e.g., "Structured Computer Organization", A. S. Tanenbaum, Prentice Hall International Editions, third edition, 1990, especially pp. 472-487.